1. Technological Field
The present invention relates to a method and apparatus for reducing vertical raster modulation in a raster scanned cathode ray tube (CRT) display.
2. Prior Art
Electron gun technology in high resolution CRT displays give very small electron beam spots on the CRT screen to provide good resolution in high addressability video formats. An increase in desired addressability stems from the increasing popularity of graphical user interfaces such as the Windows operating system from Microsoft and the OS/2 operating system from IBM. However, displays are also frequently used in more conventional text modes such as the Disk Operating System (DOS) text mode. These older text modes have lower addressabilities, leading to vertical break up of characters when a display with a small electron beam spot size is used. The character break up is generally known as vertical raster modulation. Displays having aperture grille CRTs are particularly susceptible to vertical raster modulation.
Conventionally, vertical raster modulation has been avoided by "doubling" the pixels in both the horizontal and vertical directions of the raster, thereby doubling the line density of the displayed image. However, many computer graphics adaptors do not have pixel doubling capability. Consequently, when they drive a high resolution display, vertical raster modulation can be come noticeable.
Vertical raster modulation in CRT displays having a single electron beam, such as for example, monochrome CRT displays or colour displays having a "Trinitron" CRT or similar CRT (Trinitron is a trade mark of Sony Corporation), the above mentioned problem of vertical raster modulation has been solved by stretching the electron beam spot in a vertical direction. This practice is generally referred in the art as applying an astigmatism to the spot. Conventionally, the astigmatism is applied by adding a four pole radially spaced electro-magnet structure to the electron gun assembly of the CRT. The four poles are equally spaced around the circumference of the neck of the CRT to generate a magnetic field which compresses the electron beam in a first direction and stretches it in a second direction orthogonal to the first direction, depending on the direction of current flow through the electro-magnet. Because the magnetic field generated by the four pole magnet structure acts in two orthogonal directions, this arrangement is not suitable for application to conventional in-line electron gun colour CRTs. Specifically, if used with such CRTs, this arrangement would cause convergence errors in the horizontal direction.